1. Field of the Invention
This invention relates to the plural stage purification of propylene oxide. More particularly, this invention relates to a plural stage distillation process for removing contaminating quantities of impurities from propylene oxide. Still more particularly, this invention relates to a plural stage distillation process for the purification of impure propylene oxide contaminated with impurities including water, propylene, propane, acetaldehyde, methyl formate, propionaldehyde, hexenes, acetone, hexanes, methanol, t-butyl alcohol, pentanes, pentenes, isopropyl alcohol and t-butyl formate.
Contaminating quantities of hexenes, hexanes, pentenes, pentanes, water and oxygen-containing impurities are removed from impure propylene oxide by fractionating the impure propylene oxide in a first distillation column to provide a first overhead fraction comprising hydrocarbons, including essentially all of the pentenes and pentanes and most of the oxygen-containing impurities boiling above propylene oxide and a partially purified first propylene oxide bottoms fraction comprising propylene oxide, hexenes, hexanes, and only residual quantities of pentenes and pentanes, extractively distilling the first propylene oxide bottoms fraction in a second extractive distillation column in the presence of a C.sub.2 to C.sub.6 alkylene glycol extractive distillation agent to form a second overhead fraction comprising propylene oxide, hexenes, hexanes, water, methanol, residual quantities of pentenes and pentanes and oxygen-containing impurities boiling above propylene oxide, and a second bottoms fraction comprising the extractive distillation agent, water, methanol and oxygen-containing impurities boiling below propylene oxide, extractively distilling the second overhead fraction in a third extractive distillation column in the presence of a C.sub.7 to C.sub.10 alkane hydrocarbon extractive distillation agent to form a third overhead fraction comprising propylene oxide and the hydrocarbon extractive distillation agent enriched with oxygen-containing impurities boiling above propylene oxide and water and a third bottoms fraction comprising the hexanes, residual quantities of other hydrocarbons and oxygen impurities and water, and extractively distilling said third bottoms fraction in a fourth extractive distillation column in the presence of a C.sub.7 to C.sub.10 alkane hydrocarbon extractive distillation agent to form a fourth fraction consisting essentially of propylene oxide and a fourth bottoms fraction comprising the extractive distillation agent, hexenes and hexanes.
2. Prior Art
It is known to use alkane hydrocarbons containing 6 to 18 carbon atoms as extractive distillation agents in the purification of propylene oxide. See, for example, Binning et al. U.S. Pat. No. 3,338,800, Jubin U.S. Pat. No. 3,464,897, Jubin U.S. Pat. No. 3,607,669 and Schmidt U.S. Pat. No. 3,843,488. Jubin U.S. Pat. No. 3,464,897 shows that an alkane such as octane is effective for the removal of 6 carbon atom alkane impurities such as 2-methyl pentane, 4-methyl pentene-1, 2-methyl pentene-1 and 2-methyl pentene-2. Schmidt U.S. Pat. No. 3,843,488 shows that alkanes containing from 8 to 20 carbon atoms, and preferably from 8 to 10 carbon atoms, such as n-octane, is effective for removing hydrocarbon impurities containing 5 to 7 hydrocarbons from propylene oxide.
It is also known to use alkylene glycols containing from 2 to 6 carbon atoms as extractive distillation agents in the purification of propylene oxide. See, for example, Washall U.S. Pat. No. 3,578,568, Kageyama et al. U.S. Pat. No. 3,838,020, Shih et al. U.S. Pat. No. 5,000,825, Marquis et al. U.S. Pat. No. 5,139,622, and Marquis et al. U.S. Pat. No. 5,160,587. Thus, Washall U.S. Pat. No. 3,578,568 discloses the use of ethylene glycol and propylene glycol as extractive agents for the purification of propylene oxide. Kageyama et al. U.S. Pat. No. 3,838,020 discloses the use of butylene glycols for this purpose. Shih et al. U.S. Pat. No. 5,000,825 discloses the use of glycols containing 2 to 4 carbon atoms such as ethylene glycol, propane diol, butane diol, etc. Marquis et al. U.S. Pat. No. 5,139,622 discloses the use of triethylene glycol as an extractive distillation agent and Marquis et al. U.S. Pat. No. 5,160,587 discloses the use of dipropylene glycol as an extractive distillation agent.
The use of plural stage distillation for the purification of propylene oxide has also been proposed.
Schmidt U.S. Pat. No. 3,881,996 discloses a plural stage process for the purification of propylene oxide including a first fractionation wherein light impurities such as acetaldehyde are removed overhead followed by a second distillation step wherein heavy impurities such as propion-aldehyde are removed in order to provide a second distillate fraction which is then extractively distilled in the presence of octane in a third distillation zone to provide pure propylene oxide and to remove alkane hydrocarbon impurities such as C.sub.6 carbon atom impurities from the propylene oxide. Schmidt et al. teach that it is important to use the proper sequence of distillation steps and that, for example, removal of heavy impurities such as propionaldehyde before the removal of light impurities such as acetaldehyde will lead to adverse results.
Shih U.S. Pat. No. 5,133,839 discloses a plural stage process for the purification of propylene oxide utilizing a conventional distillation zone which preferably contains two distillation columns, as in Schmidt U.S. Pat. No. 3,881,996, followed by extractive distillation of the parts of purified propylene oxide in two sequential extractive distillation columns using either isoctane or a lower alkylene glycol as the extractive distillation agent and removing lighter impurities in the first of the extractive distillation columns and heavier impurities from the second of the extractive distillation columns.
Meyer et al. U.S. Pat. No. 4,971,661 discloses a plural stage process for the purification of propylene oxide, but different extractive distillation agents such as water and acetone are used.